1. Field of Invention
The invention relates to a data phase locked loop (PLL) circuit, and, in particular, to a data phase locked loop circuit used in an optical drive and a method of generating a frequency of a reference signal.
2. Related Art
With the rapid development of information processing and electronic technology, various electronic circuits, such as filters, phase locked loop circuits and other devices for processing various signals are continuously invented and improved. The phase locked loop circuit is a powerful device that can be applied to synchronous processes, frequency dividing processes or frequency multiplying processes. In addition, the phase locked loop circuit may be implemented by using a cheap integrated circuit. Thus, the phase locked loop circuit is widely used in the signal processing in, for example, an optical drive.
FIG. 1 shows a conventional data phase locked loop circuit 10. The data phase locked loop circuit 10 includes a phase locked loop circuit 11, a protecting circuit 12 and a detecting circuit 13. A data signal SD is read from an optical disk 100 and is inputted to the phase locked loop circuit 11 by an optical pickup head 101 of an optical drive. Then, a reference signal SR is generated according to the data signal SD and is outputted to an optical drive signal processing circuit 102 by the phase locked loop circuit 11. The reference signal SR may serve as a signal processing reference for the optical drive signal processing circuit 102. The data signal SD is continuously tracked to equalize the frequency of the reference signal SR to the data signal SD by the phase locked loop circuit 11. The condition of the optical pickup head 101 is detected in reading the optical disk 100 according to a radio frequency signal RF obtained by the optical pickup head 101, and then a control signal SC is outputted to the protecting circuit 12 by the detecting circuit 13. Next, the protecting circuit 12 decides whether to output a sustaining signal SS to the phase locked loop circuit 11 according to the control signal SC.
However, when the optical pickup head 101 reads a defect zone of the optical disk 100, the amplitude of the data signal SD fluctuates severely or cannot be read. Therefore, the frequency of the reference signal SR is too high or the frequency of the reference signal SR cannot be predicted, thereby disabling the optical drive signal processing circuit 102 from operating normally. In order to overcome the above-mentioned problem, a high-level control signal SC is outputted by the detecting circuit 13 when the detecting circuit 13 detects that a defect zone of the optical disk 100 is read, and a sustaining signal SS is outputted by the optical pickup head 101 when the optical pickup head 101 detects that the control signal SC has a high level. Thus, when the sustaining signal SS is received by the phase locked loop circuit 11, the reference signal frequency SR is fixed to an arbitrary frequency at this time. A low-level control signal SC is outputted when the detecting circuit 13 detects that the optical pickup head 101 is reading a defect-free zone of the optical disk 100 so as to disable the protecting circuit 12 from outputting the sustaining signal SS. Thus, the frequency of the reference signal SR generated by the phase locked loop circuit 11 progressively tracks from the arbitrary frequency to the frequency of the data signal. Because the arbitrary frequency is unpredictable or is too great, a longer period of delay time is needed to enable the tracking of the frequency of the reference signal SR from the arbitrary frequency to the frequency of the data signal. Thus, the optical drive signal processing circuit 102 still cannot work normally in this period of delay time.
Therefore, it is an important subject of the invention to provide a data phase locked loop circuit and a method of generating the frequency of the reference signal in the data phase locked loop circuit of the optical drive in order to shorten the delay time required to track the frequency of the reference signal from an arbitrary frequency to the frequency of the data signal, when the defect zone read by the optical drive ends, to enable the optical drive signal processing circuit 102 to recover to normal operation.